Plated materials having a plating layer comprised of tin (Sn) Sn alloy, or the like on a conductive substrate (appropriately referred to a substrate hereinafter) such as copper (Cu) and Cu alloys, are known as high-performance conductors having excellent conductivity and physical strength of the substrate as well as excellent electric continuity, corrosion resistance, and solderability of the plating layer, and have been widely used for various terminals, connectors, and the like. To this plated material, nickel (Ni), cobalt (Co), iron (Fe), or the like having a barrier function is usually underplated on the substrate, in order to prevent alloy components of the substrate (appropriately referred to substrate components hereinafter), such as Zinc (Zn), from diffusing into the plating layer.
While an oxide film is formed on the Sn plating layer on the surface of the terminal, since Sn is readily oxidized under a high temperature environment such as in an engine room of an automobile, this oxide film is so brittle that it is readily broken by connecting the terminal. Consequently, a non-oxidized Sn plating layer under the oxidized Sn plating layer is exposed to give good electric continuity.
Since a fittable connector is made into a multipolar type with the spread of electronic control in recent years, a large force is necessary for push-in/pull-out between a group of male terminals and a group of female terminals. In particular, since push-in/pull-out work is difficult in a narrow space such as the engine room of the automobile, reduction of the push-in/pull-out force is earnestly required.
As a method for reducing the push-in/pull-out force, the contact pressure between the terminals may be weakened by widening the gap between the contact points or by thinning the Sn plating layer on the surface of the connection terminal. However, according to this method, defective continuity may occur between the terminals due to a fretting phenomenon that occurs between the contact faces of the terminal since the Sn plating layer is soft.
In the fretting phenomenon, the soft Sn plating layer on the surface of the terminal is worn and oxidized, due to fine fretting between the contact faces of the terminal caused by vibration, temperature changes, and the like, to form a worn powder having a large specific resistivity. Defective continuity occurs when this phenomenon emerges between the terminals. This phenomenon is liable to emerge as the contact pressure between the terminals is lower.
For preventing the fretting phenomenon, there is proposed a method for forming a hard Cu—Sn intermetallic compound layer, such as Cu6Sn5, that hardly causes the fretting phenomenon on the substrate. However, this method involves such a problem that the Cu—Sn intermetallic compound layer becomes so brittle by diffusion of a large amount of the substrate components, such as Cu, into the Cu—Sn intermetallic compound layer.
The plated material, in which diffusion of substrate components is prevented by providing a Ni layer between the substrate and the Cu—Sn intermetallic compound layer, may be manufactured by sequentially plating layers of Ni, Cu, and Sn, in this order on the substrate, since neither the Sn layer nor the Cu layer is formed between the Ni layer and the Cu—Sn intermetallic compound layer. When this plated material is subjected to heat-treatment, the thickness of each plating layer of the plated laminate should be strictly designed based on chemical stoichiometric ratio of Cu and Sn with thorough control of heat-treatment. Therefore, production of such a plated material required much labor.